1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, and a processing apparatus which is employed by, e.g., an image forming apparatus.
2. Discussion of Related Art
There has been known an image forming apparatus, such as a laser printer, that includes a photosensitive drum, and an electrifier, a scanner, a developing roller, and a transferring roller all of which are provided in the order of description around the photosensitive drum in the direction of rotation of the drum. As the photosensitive drum is rotated, first, an outer circumferential surface of the drum is uniformly electrified, i.e., electrically charged by the electrifier, and subsequently the outer surface is exposed to a high-speed scanning of a laser beam emitted by the scanner. Thus, electrostatic latent images corresponding to image data are formed in the outer surface of the photosensitive drum. Then, as the developing roller is rotated while being engaged with the photosensitive drum, toner carried on an entire outer surface of the developing roller is selectively attracted by only the electrostatic latent images formed in the outer surface of the drum, and thus carried on the outer surface of the same. Thus, toner images are formed on the photosensitive drum. Subsequently, as the photosensitive drum is rotated while being engaged with the transferring roller via a recording sheet, the toner images carried on the outer surface of the drum are transferred to the recording sheet being fed forward by the cooperation of the drum and the roller.
There haven been known two types of transferring rollers each of which can be employed by the above-described image forming apparatus, i.e., an electron-conductive-type roller and an ion-conductive-type roller.
The electron-conductive transferring roller includes a roller portion formed of an elastic material in which electrically conductive particles or fillers are dispersed, and is characterized in that a resistance value of this transferring roller is less changed by the environment, but variations among individual rollers and variations among respective axial portions of each roller are greater and accordingly this roller suffers from a disadvantage that the image transferring operation of the roller cannot be controlled with high stability.
On the other hand, the ion-conductive-type transferring roller includes a roller portion formed of an elastic material to which an ionic substance is added, and is characterized in that a resistance value of this transferring roller is more changed by the environment, but variations among individual rollers and variations among respective axial portions of each roller are smaller and accordingly this roller enjoys an advantage that the image transferring operation of the roller can be controlled with high stability. Thus, the ion-conductive transferring roller is widely used.
However, the ion-conductive transferring roller has the above-described disadvantage that the electric resistance of the roller is greatly changed by the environment. Hence, Japanese Patent Application Publication No. P2003-5614A discloses an image forming apparatus in which a bias is applied to an ion-conductive transferring roller so as to measure a resistance value of the roller, a temperature of the roller is calculated based on the measured resistance value, and a rotation number and a rotation time of a cooling fan are controlled based on the calculated temperature of the roller, i.e., a temperature of the air around the roller.
Thus, in the image forming apparatus disclosed by the above-indicated document, the rotation number and time of the cooling fan are controlled to adjust the temperature of the air around the ion-conductive transferring roller. However, the temperature of the air around the roller cannot be satisfactorily controlled by just controlling the rotation number and/or time of the fan.
Described in more detail a degree of cooling of the transferring roller can be changed by increasing or decreasing the rotation number and/or time of the cooling fan. However, for example, when the image forming apparatus is turned on in a low-temperature environment, the transferring roller cannot be positively heated and accordingly it is difficult to increase quickly and accurately the temperature of the roller up to a required level.
In addition, in the image forming apparatus disclosed by the above-indicated document, the rotation number and time of the cooling fan are finely controlled based on the measured resistance of the transferring roller. Therefore, a complicated control method is needed.